Certain gas turbine components, such as, for example, shaft-type labyrinth seals and rotating blade shroud seals, are often constructed from honeycomb structures of thin foils of high-temperature metal alloys. The honeycomb structure provides a high structural strength combined with a low weight. The honeycomb structure also simultaneously provides an excellent sacrificial wear surface and an excellent gas flow seal. The spacing of the honeycomb cell is typically in the range of about 0.8 mm to about 3 mm (about 0.031 in. to about 0.125 in.) across. The depth of the honeycomb cell is typically in the range of about 1.5 mm to about 21.6 mm (about 0.062 in. to about 0.85 in.) but may be less than or greater than this range depending on the application.
The honeycomb structure in a turbine seal allows for the expansion of turbine materials during turbine operation with increasing temperature and centrifugal force. The honeycomb structure is often brazed to a backing structure. The brazing joins the metal foils of the honeycombs to the metal of the backing structure. A sacrificial layer of a honeycomb structure of a relatively soft material brazed into a wide slot built into the turbine wall along the blade path allows the blades to grow. The harder blade tips cut a path into the softer honeycomb shroud seal, which preserves the integrity of the blades and the turbine wall.
In order to achieve the desired wear surface, the honeycomb material must be softer than the blade material. As the braze material tends to be harder than the honeycomb material after brazing, it is important to minimize the amount of braze material at the wear surface of the honeycomb to avoid wear on the blade. It is difficult, however, to apply the braze material to the base of the honeycomb for brazing to the backing structure due to the structure of the honeycomb.
Braze tape, braze powder, and braze paste can each be used for brazing a honeycomb structure to a backing structure, but each has its disadvantages. Although braze tape applies an even, minimal amount to the metal foils at the base of the honeycomb, the braze tape spans the empty space between metal foils where braze composition is not needed or desirable, and is therefore a waste of braze material. Braze tape is also expensive and must be handled carefully to prevent detachment prior to brazing. Braze powder can be added to the honeycomb cells by peppering by hand or by an automated chute, but a layer of powder that also covers the entire empty space of each honeycomb cell must be applied, and a cement must then be sprayed over the braze powder to hold it in place prior to brazing. Finally, the viscosity of a braze paste makes it difficult to provide a small, consistent amount of braze paste to the base of the honeycomb, and a braze paste may adversely affect the tack weld quality.